Abarim Publications' online Biblical Hebrew Dictionary
The Semitic root קנא (qn') appears to denote a becoming deep red (it does so in Arabic), and as such spawns words all over the Semitic language spectrum that have to do with jealousy. This probably has nothing to do with getting an angry red face, but rather with jealousy being a defining quality of life. In other words: jealousy is right up there with love and hate, although in our stunted modern world it's not often recognized as such (see our article on the damam-words which includes the name Adam and the noun דם, dam, meaning blood).
In Hebrew the derived feminine noun קנאה (qin'a) means ardor, zeal or jealousy. Unlike someone who steals out of hunger, someone who copulates with his neighbor's wife can not count on understanding and forgiveness but can count on wounds and disgrace, because jealousy enrages a man (Proverbs 6:34, 27:4).
Jealousy was recognized to be such a strong emotion that a substantial piece of Mosaic legislation dealt specifically with the jealous feelings of a husband towards his supposedly unfaithful wife (Numbers 5:11-31). Still, the Preacher recognized that every labor and every skill which is done comes from rivalry between men (Ecclesiastes 4:4), and lists zeal right along love and hate (Ecclesiastes 9:6).
The exact same word serves to describe a person's zeal for righteousness, which obviously taps into the greater marriage-and-adultery theme that explains the relationship between YHWH and mankind (Ezekiel 16:38). When Phinehas gored Zimri and Cozbi, he did so because he was jealous with the jealousy of the Lord (Numbers 25:11). Subsequently, the Lord offered him and his descendants his covenant of peace (Numbers 25:12) and perpetual priesthood because he was jealous for his God and made atonement for the sons of Israel (Numbers 25:13). It is assumed that Israel's nationalistic movement of the Zealots derived their name from this statute.
The military captain Jehu's zeal for the Lord appears to have prompted him to embark on an unequaled killing spree (2 Kings 10:16) and David famously exclaimed, "Zeal for Thy house has consumed me..". (Psalm 69:9). But more often, the Lord's zeal for his people alters history (2 Kings 19:31, Isaiah 9:6), as he arouses his zeal like a man of war (Isaiah 42:13, Zechariah 1:14, 8:2), as he wraps himself with zeal like a mantle (Isaiah 59:17).
Sometimes our noun is used in such a way that it expresses a sentiment rather akin to anger. The Lord God declares that he will deal with the men of Seir according to their anger (Ezekiel 35:11), but Eliphaz assured Job that the simple will be dealt with by their own anger (Job 5:2), as anger rots the bones (Proverbs 14:30). But mostly, it's God's anger that's coming the way of wayward men (Deuteronomy 29:20, Ezekiel 5:13, Zephaniah 1:18, Psalm 79:5).
Our noun also yields the denominative verb קנא (qana'), meaning to be jealous or zealous, pretty much with the same compass as the noun (Genesis 30:1, 2 Samuel 21:2, Isaiah 11:13) or to provoke or entice jealousy or zeal (Deuteronomy 32:16, 1 Kings 14:22, Ezekiel 8:3).
Two adjectives derive from this root. Both mean jealous and both are only applied to God: namely קנוא (qanno') (Joshua 24:19, Nahum 1:2) and קנא (qanna) which is used in the Lord's important confession that he is a jealous God (Exodus 20:5 and Deuteronomy 5:9, also see Deuteronomy 4:24 and 6:15). In Exodus 34:14 it's even stated that the Lord's name is Jealous God (El-kana).